Urban Stormwater Management: Can Tree Roots and Structural Soils Improve Hydraulic Conductivity into Compacted Soils?

Typically, surface precipitation runoff is a key source of flooding and water pollution in urban communities, and the costly and time-consuming process of installing bio-retention basins is one approach to overcoming these challenges. The implementation of structural soils in bio-retention basins designed to receive and retain stormwater provides these systems with additional functions to bear loads and facilitate tree root growth and exploration. The channels that tree roots produce as they grow can also aid in the flow of water down the soil profile. In this study, the potential for tree roots to penetrate compacted soils and increase rates of hydraulic conductivity were examined alongside the use of structural soil in the context of urban stormwater systems. For the first experiment, Pouteria obovata and Calophyllum soulattri together with a control (without tree) were placed in cylindrical planting sleeves surrounded by compacted clay loam at two compaction levels (bulk densities of 1.45 g cm−3 and 1.66 g cm−3). Roots of both species penetrated the compacted soil, and hydraulic conductivity was increased by an average of 50%. In the second experiment, the same species were grown in structural soil, and a geotextile separated the compacted soil (bulk density of 1.66 g cm−3) from the structural soil (compacted). A greater number of roots as well as larger root diameters from Pouteria obovata penetrated the geotextile, and hydraulic conductivity was enhanced twofold when compared to the controls that had no trees. Growing woody rooting plants and installing structural soils within urban stormwater systems may confer benefits of increased water infiltration and enhanced root development, alongside potential overall improvements to tree health for stormwater control systems in urban environments.